Light coupled into and out of single-mode fibers generally requires some form of conversion to minimize insertion losses. One such conversion involves collimating light at fiber ends to expand mismatch tolerances, particularly longitudinal mismatch.
Telecommunication and sensor systems rely on single-mode optical fibers as favored medium for conveying optical information, particularly over long distances. Coupling light into a single-mode fiber (e.g., connecting the fiber to a source) and coupling light out of a single-mode fiber (e.g., connecting the fiber to a detector) involve well-established technologies. However, such couplings remain expensive and complex because of extremely high tolerances for alignment, cleanliness, and dimensioning of the coupling system.
Mid-link couplings, which interrupt single-mode fibers along their length, support functions such as switching, routing, and signal modification or restoration. More such couplings are needed to accommodate a rising demand on fiber optic systems to perform increasingly complex tasks. The additional couplings add considerable cost and complexity to the fiber systems.
A typical single-mode fiber coupler includes a bulk optical lens, such as a gradient-index (GRIN) lens, fused to one end of a single-mode fiber section. The lens converts light diverging from a core of the fiber end into a more collimated form for further propagation through free space or another optical component. Another lens can be coupled to an end of a second single-mode fiber section for collecting the collimated light and for converging the light onto a core of the second fiber section. Although such lenses contribute to expanding longitudinal and transverse assembly tolerances for connecting single-mode fibers to or from free space and any intervening optics, the lenses themselves must also be aligned, which involves similarly tight tolerances.
Tapered couplers, which can be formed near the ends of single-mode fibers, shift light traveling in a single-mode core (i.e., a core mode of transmission) into the lowest order mode of a multi-mode, expanded-core region so that light exits the fibers substantially more collimated. The larger diameter modes of the expanded-core region are inherently more collimated than the smaller diameter mode of the single-mode core region. The mode shifts are accomplished by adiabatically tapering the core to enable light traveling along the core to remain in the lowest order mode as the core size increases and hence supports multiple modes. Similar couplings can be used to collect collimated light and shift the collimated light from a large-diameter mode in a multiple-mode region to the small-diameter mode of the single-mode core region for further propagation along a single-mode fiber.
Such tapered couplings efficiently convert light between the core mode and the lowest order mode of a multi-mode section, but manufacturing these tapered couplings to required specifications can be difficult. Fiber ends are carefully drawn down at elevated temperatures and cleaved to achieve the required form. The cost of such operations is high, particularly as an incremental cost repeated over many such couplings.
Lower cost couplings for single-mode optical fibers with additional performance options can be realized by using grating structures to convert light between core and cladding modes near fiber ends. Here, cladding modes refer to the modes that are guided within the cladding structure that surrounds the core of a standard single-mode fiber. The use of gratings for mode conversion improves coupling possibilities for light entering and exiting the ends of single-mode fibers.
A single-mode optical fiber incorporating an exemplary mode-converting coupler includes a core surrounded by a cladding and an end adapted for coupling the fiber to a continuing optical pathway. A grating formed near the end of the fiber shifts transmissions of light between the core and the cladding so that the light passing through the fiber end is substantially more collimated, thereby expanding both transverse and longitudinal alignment tolerances for coupling the fiber to the continuing optical pathway.
The grating can be formed by a pair of reflective short-period gratings, such as a Bragg gratings, or by a transmissive long-period grating. One of the reflective gratings is written into the core of the single-mode fiber for performing the mode conversion, and the other reflective grating is written into the cladding of the single-mode fiber for redirecting the light toward the fiber end. The transmissive grating performs the same mode conversion without requiring a change in direction. Reflective gratings have the advantage of requiring a minimum of space for reflecting narrow spectral bands, but reverse direction. Transmissive gratings require more space to achieve a similarly specific spectral response but do not reverse direction.
Both types of gratings can be formed directly in the single-mode fibers and subsequent assembly is not required. In addition, manufacture of the gratings can be accomplished without any drawing or furnace operations. Reflective gratings can be written by exposing a photosensitive core or cladding to actinic UV radiation in the form of an interference pattern. Transmissive gratings can be written in a similar fashion, or by using a uniform beam and a periodic shadow (or amplitude) mask, or by successively exposing relatively translated lengths of the fiber. Preferably, both types of gratings are formed near the fiber ends, and perturbations (i.e., grating lines) are written into the fiber at orientations that are substantially normal to the direction of light propagation for minimizing losses of light through sides of the fiber.
The mode-converting couplings can be made to free space; to other fibers or waveguides; or to optical devices such as lasers, detectors, planar or bulk optics, and micro-electro-mechanical systems (MEMS). Grating structure can be used to convert light between the fundamental core mode and one or more higher cladding modes. The mode-converting couplings can also be used as pre-collimators in combination with bulk optical lenses to meet more stringent collimating requirements. Simpler (e.g., less fast) bulk lenses can be used because of the pre-collimating function of the mode-converting couplings.